1. Technical Field
The present invention relates to antenna testing, and more particularly, to a method and apparatus for testing the Voltage Standing Wave Ratio (VSWR) to determine performance characteristics of an antenna and cabling connected to the antenna.
2. Description of Related Art
FIG. 1 shows a testing apparatus generally indicated as 10 known in the art for testing the VSWR of one or more receive antennas generally indicated as 12 and cabling associated with a typical cellular base site. The testing apparatus 10 includes test equipment 14 and two directional couplers 16, 18 for coupling between the antenna 12 to a receive multi-coupler (RMC) 20.
An antenna connector 22 couples the antenna 12 to the directional coupler 16. A second directional coupler 18 couples the directional coupler 16 to the receive multi-coupler (RMC) 20. The antenna connector 22 and the directional couplers 16, 18 provide a signal path between the antenna 12 and the receive multi-coupler 20 as shown by a signal line 23.
The receive multi-coupler 20 includes a filter 24, an amplifier 26 and a splitter 28. The filter 24 filters the antenna signal received by the antenna 12 and provides a filtered signal. The amplifier 26 amplifies the filtered signal and provides an amplified and filtered antenna signal. The splitter 28 splits the amplified and filtered signal into several component parts and provides split signals to receivers 30, 32.
The test equipment 14 includes a test signal source 40, two receivers 42, 44 and a signal processor 46. The test signal source 40 provides a test signal indicated as 41 to the directional coupler 18. The directional coupler 16 provides one part of the test signal, known as a forward signal indicated as 41a, to the receiver 44. The test signal 41 reflects off the antenna 12, and the directional coupler 16 provides a reflected part of the test signal, known as a signal indicated as 41b, to the receiver 42. The receivers 42, 44 respectively measure the amplitude and the signal strength of the forward and reverse signals and provide this measurement information to the signal processing circuit 46. The signal processing circuit 46 processes the measurement information, and provides signal processing with an indication of the antenna reflection coefficient and the VSWR of the antenna 12.
The VSWR is an important factor that affects the performance characteristics of an antenna and provides important information about how the antenna will operate. If there is a mismatch of impedance along a circuit including a transmitter or receiver, transmission line and an antenna, there will be an inefficient transfer of energy either from the transmitter via the transmission line to the remote wireless receiver, or from the remote wireless transmitter via the antenna and the transmission line into the receiver. Because of the impedance mismatch, not all of the energy will flow forward from the transmitter to the antenna or similarly from the antenna to a receiver.
When an electromagnetic wave is sent down a transmission line which is not terminated with its characteristic impedance, some of the energy is reflected back from the termination. In some locations along the transmission line, the reflected wave re-enforces the direct wave. In other locations along the transmission line, the reflected wave opposes the direct wave. The result is a stationary pattern of waves along the line called standing waves. The VSWR is the ratio of the maximum to minimum voltage of a standing wave along a transmission line.
More particularly, the standing waves created in a transmission line are standing waves of voltage and current on the line. The voltage maximums and minimums of a standing wave have a constant amplitude. The ratio of the maximum to minimum voltage on a line is called the VSWR and is given by the following equation: EQU VSWR=V.sub.1 max/V.sub.1 min
If the source and load impedances match that of the transmission line, there are no standing waves and the VSWR is 1. However, if there is an impedance mismatch between the transmission line and the source and load impedances, the VSWR will be greater than 1.
While the above equipment and method for testing the characteristics of the antenna 12 provides a generally reliable and accurate indication of the antenna and cabling signal transmission characteristics, this test equipment is very expensive to implement because of the requirement of the directional couplers and signal lines. One problem with the known test equipment is that it requires two directional couplers, receivers, and associated cabling, a first for providing the test signal, and a second for returning the unreflected and reflected signals back to the equipment. Directional couplers and associated cabling are expensive (about $150++ depending on quantity) per antenna, and there are a number of antennas per cellular site. Therefore, a method and apparatus for testing the VSWR of an antenna, which is easy and more cost effective to implement, is needed.